Solid lithium-ion electrolyte

ABSTRACT

The present invention relates to the composition of a solid lithium-ion electrolyte based on the Li 2  O--CeO 2  --SiO 2  system having good transparent characteristics and high ion conductivity suitable for uses in lithium batteries, electrochromic devices and other electrochemical applications.

CONTRACTUAL ORIGIN OF THE INVENTION

The United States Government has rights in this invention under contractnumber DE-AC36-83CH10093 between the U.S. Department of Energy and theNational Renewable Energy Laboratory, a Division of the Midwest ResearchInstitute.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to an inorganic, solid lithium-ionelectrolyte material based on the Li₂ O--CeO₂ --SiO₂ system that hasuses in primary and secondary batteries, electrochromic coatings andother electrochemical devices.

2. Description of the State of the Art

Ionic electrolytes are electronic insulators that are also excellentconductors of a specific ion. They are used in electrochemical devicessuch as primary and secondary (rechargeable) batteries andelectrochromic windows and displays and in electrochemical sensors. Inelectrochemical devices, the mobile ions may migrate through theelectrolyte under the influence of a chemical potential or under theinfluence of an applied electrical potential. In the first case, thecomplementary, external electronic current is commonly used to providepower as from the discharge of a battery or to produce an electricalsignal as in a sensor. In the second case the mobile ions are typicallydriven from one material into another either to store energy as incharging a secondary battery or to change the properties of a device asin switching an electrochromic device from transparent to opaquecondition.

Typical solid electrolytes are metal oxides with mobile ions such ashydrogen, lithium, sodium or oxygen. The electrolytes are required to besolid in applications where the electrochemical device must operate attoo high a temperature for liquid electrolytes to be used or where thinfilms of the electrolyte must be used. Examples of common uses for solidelectrolytes include high-temperature zirconium oxide oxygen-ionconductors used in oxygen sensors for controlling the fuel/air mixtureratio in automobile engines; sodium-beta-aluminum oxide sodium-ionconductors used in sodium-sulfur secondary batteries above 300° C.; andlithium-doped cobalt oxide solid lithium-ion conductors used in primaryand secondary lithium batteries. Solid electrolytes are also used inelectrochromic devices where they must also be optically transparentthin films.

Lithium-ion conductors are preferred as solid electrolytes in severalapplications because they generally combine high ionic conductivity withgreater chemical stability than other fast ion conductors such ashydrogen-ion conductors. They can also produce higher voltages insolid-state batteries than can analogous hydrogen-ion conductors. J. B.Goodenough has reviewed the characteristics of fast ionic conduction insolids in the Proceedings of the Royal Society, A 393, pages 215-234,1984.

A U.S. Pat. No. 5,384,653 to Benson et al discloses a stand-alonephotovoltaic powered electrochromic window. The electrochromic windowincludes in its electrochromic coating a solid thin film electrolyteformed of a fast-ion conductor material in which either lithium orhydrogen ions diffuse readily.

Examples of such fast ion conductor materials include: Li₃ N, Li₂ NH,and Li_(1-X) M_(X) Ti_(2-X) (PO₄)₃, where M represents another metalsuch as aluminum, chromium, gallium, or the like. Such solid thin filmelectrolyte Myers are typically 1,000 to 5,000 Å thick as disclosed inBenson et al. Although, solid thin film lithium ion conductor materialis known as disclosed in Benson et al, they have not demonstratedoptimum properties for such electro-optic devices. For example, the Li₃N is not completely transparent and most other solid lithium-ionconductors are not sufficiently conductive at room temperature or arenot sufficiently stable against attack by atmospheric gases includingwater vapor.

U.S. Pat. No. 4,768,865 to Greenberg et al also discloses anelectrochromic device having an ion-conductive layer. In this patent,the ion conductive material may be solid, liquid or gel but ispreferably disclosed as being a polymer layer.

Lithium ion conductors based on lithium silicate have been investigatedby several groups. Most of the previous works focus on the bulkproperties of ion-conducting glasses or gels. For example, lithiumsilicate gel has been reported to have an ionic conductivity of 10⁻³S/cm at 300° C. (Wakamatsu et al, "Effect of Lithium Salts on the IonicConductivity of Lithium Silicate Gels," Journal of Non-CrystallineSolids 147/148, 1992, 668-671). Sol-gels of the Li₂ O--P₂ O₅ --SiO₂system have also been used to produce films of solid lithium-ionelectrolytes (Hayri et al, "The Preparation and Ionic Conductivity ofSol-gels in the LiO₂ --P₂ O₅ --SiO₂ System," Journal of Non-CrystallineSolids 94, 1987, 387-401). The publication acknowledges that manyattempts were also made to prepare potassium analogs but that all wereunsuccessful.

Xerogels of (LiCl)₂ --R₂ O₃ --SiO₂ (R═B, Al, Ga) have also been prepared(Wang et al, "Ionic Conductivity of (LiCl)₂ --R₂ O₃ --SiO (R--B, Al, Ca)Xerogels," The Physics of Non-Crystalline Solids, Vol. 203, 1992,204-206).

Recently, efforts have been made to prepare lithium silicate in thinfilm forms (Mouchon et al, "Sol-gel Lithium Silicate Electrolyte ThinFilms," Material Res. Soc. Symp. Proc, Vol. 346, 1994, 189-199), but noionic conductivity data were reported for these thin films.

The French patent, 26 33609, to Arribart et al discloses the use of acerium oxide counter electrode for electrochromic applications. A TiO₂--CeO₂ system has also been proposed to replace CeO₂ as thecounter-electrode in electrochromic devices. The use of CeO₂ has adisadvantage in that its electrochromic reaction kinetics are slow.

A U.S. Pat. No. 5,387,330 to Taniguchi et al further discloses amixed-oxide solid hydrogen-ion and oxygen-ion mixed ionic conductor foruse in electrochromic devices, fuel cells and sensors. The patent isspecifically directed to a mixed ion conductor having the compositionBaCeO_(x) Gd_(y) O_(3-a).

SUMMARY OF THE INVENTION

It is the object of the instant invention to provide an improved solidlithium-ion electrolyte that has high visible transparency and highlithium-ion conductivity with a low electronic conductivity.

It is a further object of the instant invention to provide a threecomponent system lithium-ion conductor that is useful in electrochromicdevices as well as battery, and other electrochemical devices.

It is an additional object of the instant invention to provide alithium-ion conductor with optimum lithium-ion transport qualities.

The instant invention is directed to a solid lithium-ion electrolytebased on the Li₂ O--CeO₂ --SiO₂ system. Thin films of this material havebeen prepared by the sol-gel technique. The lithium-ion conductor isalso highly transparent, making it very suitable for use inelectrochromic devices.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are incorporated in and form a part of thespecification together with the description to explain the principles ofthe invention. The drawings are illustrative and the various elementsare not to proper scale.

FIG. 1 is the film-preparation flow chart for a typical solid ionicelectrolyte film.

FIG. 2 is a schematic of an electro-optical measurement set-up forhalf-cells comprising an ion conductor film (solid ion electrolyte)deposited on top of a coated glass substrate.

FIG. 3 depicts in graph form the electro-optical properties of ahalf-cell with an ion conductor film (˜500 nm thick) deposited on a 900nm WO₃ /ITO substrate. The film is formed of 20% Li₂ O--20% CeO₂ --60%SiO₂ (molar) and is colored/bleached at ±1.5 V.

FIG. 4 illustrates the optical transmittance spectra of a solid lithiumion electrolyte film deposited on a Pyrex glass substrate. The film is500 nm thick and is formed of 20% Li₂ O--20% CeO₂ --60% SiO₂ (molar).

FIG. 5 depicts in graph form the electro-optical properties of a solidlithium-ion electrolyte film measured by cyclic voltammetry. The film is500 nm thick, and is formed of 10% Li₂ O--30% CeO₂ --60% SiO₂ (molar)deposited on an ITO substrate (without a WO₃ film between the solidelectrolyte and ITO).

FIG. 6 depicts in graph form the electro-optical properties of ahalf-cell with a lithium-ion electrolyte film measured by cyclicvoltammetry. The film is 500 nm thick, and is formed of 10% Li₂ O--30%CeO₂ --60% SiO₂ (molar) deposited on a 900 nm WO₃ /ITO substrate.

FIG. 7 depicts in graph form the electro-optical properties of ahalf-cell with a lithium-ion electrolyte film that is 500 nm thick andis formed of 30% Li₂ O--20% CeO₂ --50% SiO₂ (molar) deposited on a 900nm WO₃ /ITO substrate.

FIG. 8 depicts in graph form the electro-optical properties of ahalf-cell with a lithium-ion electrolyte film that is 500 nm and formedof 30% Li₂ O--10% CeO₂ --60% SiO₂ (molar) deposited on a 900 nm WO₃ /ITOsubstrate.

FIG. 9 illustrates in cross-section transmissive electrochromic deviceusing the ion conductor layer in accordance with the instant invention.

FIG. 10 illustrates in graph form the electro-optical properties of anelectrochromic device using the ion conductor of Example 5 and theconstruction of FIG. 9. The device has a structure of glass/SnO₂ /50 nmLi--600 nm WO₃ /500 nm of ionic conductor/90 nm V₂ O₅ /12 nm Au.

FIG. 11 illustrates in cross-section a reflective electrochromic deviceusing the ion conductor of the instant invention.

FIG. 12 illustrates the optical reflectance spectra of the reflectiveelectrochromic device of FIG. 11 in both the bleached and coloredstates. The device has a structure of glass/SnO₂ /50 nm Li--700 nm WO₃/500 nm of ionic conductor/100 nm V₂ O₅ /40 nm Au, where the ionconductor described in Example 1 was used.

DETAILED DESCRIPTION OF THE INVENTION

Silicon dioxide forms a glass which is a good amorphous matrix toaccommodate other metal oxides. Cerium oxide, when added to the glass,modifies the silicon dioxide matrix, making it more open and facilitatesthe migration of small ions such as lithium. The modified silicate glassstructure offers many vacant sites that can be occupied by the mobileions and can serve as preferred pathways for ion migration. The additionof Li₂ O provides a large lithium ion background concentration andfacilitates the lithium-ion transport.

Ion conductors based on the mixed oxides of lithium, cerium and silicon,(Li₂ O--CeO₂ --SiO₂) are prepared by the sol-gel method. This is amethod wherein an appropriate solution is prepared and coated on asubstrate to form a film. The resulting film is then dried at anelevated temperature. The source materials may typically include lithiumnitrate (LiNO₃), ammonium-cerium-nitrate (Ce(NH₄)₂ (NO₃)₆), andtetraethoxysilane (Si(OC₂ H₅)₄ or TEOS). The lithium source (LiNO₃) canbe replaced by either LiCl or LiBr. The amount of water added affectsthe time from solution preparation to final solid gel formation;increasing the water content increases the time for solid formation.

The preparation procedure for a typical solid electrolyte film isdepicted in FIG. 1. The following description is for a 20% Li₂ O--20%CeO₂ --60% SiO₂ by molar electrolyte although the basic procedure wasused for all examples. The lithium nitrate, (0.04 mol), andammonium-cerium-nitrate, (0.02 mol), were dissolved in water, (1.8 mol),as the solution 1 depicted at 21. Tetraethoxysilane, (TEOS), (0.06 mol)is dissolved in the same volume of ethanol and forms the solution 2depicted at 22. The two solutions were then poured together and mixed asindicated at 23 with a magnetic stirrer at 50° C. for several hours. Theresulting transparent solution was used as the precursor for sol-gelfilm deposition.

The resulting lithium-cerium-silicon mixed oxide precursor was coated onsubstrates by the well known spin-coating method 24 using typical speedsand times of 2000 rpm and 30 seconds, respectively. The resulting filmswere slowly dried in air for about 30 minutes, then baked, 25, at 200°C. to 300° C. for approximately 10 minutes in either ambient air oroxygen environments. A typical film prepared under these conditions isabout 500 nm thick. It is understood that the basic spin coating processis known and that conditions such as the precursor viscosity, spin speedand time can be varied to control the thickness of the film. Also, othercoating processes, such as sputtering, evaporation and dip coating canbe used. These are only representative and it is understood thatadditional known coating processes are also applicable.

These films are robust when subjected to either mechanical scratch orchemical corrosion testing. They are quite stable in both basic andacidic solutions.

The ionic conductivities of the films were measured by electrodeimpedance spectroscopy using a frequency range of 5 MHz to 50 kHz. Theionic conductivity of the prepared films was on the order of 10⁻⁶ S cm⁻¹at 20° C. when prepared at bake out temperatures within the range of200°-300° C.

The electro-optical properties of Li₂ O--CeO₂ --SiO₂ films wereinvestigated in various configurations. Thin films were deposited ontransparent conductor (TC) coated glass substrates or on WO₃ /TChalf-cells. They were tested in an electrochemical cell as shown in FIG.2. The electrochemical cell 1 of FIG. 2 includes a 1 molar solution,(depicted at 2) of LiClO₄ in propylene carbonate as a lithium-ionsource. Lithium metal is used as both counter electrode 3 and referenceelectrode 4. The thin film ion conductor shown at 5 was deposited on WO₃(depicted at 6) on an ITO or SnO₂ -coated substrate 7. The coated glasssubstrate is formed by pre-coating with a transparent conductor (TC)layer of tin-doped indium oxide (ITO) or tin oxide (SnO₂) which in turnis covered by the layer of electrochromic tungsten oxide (WO₃) 6. Thesesubstrate designs are denoted as WO₃ /TC substrates. The opticaltransmittance of the half cells were measured by using a diode laser,(8), having a wavelength of ˜788 nm or 670 nm in conjunction with thelight detector (9). The device power was provided by a potentiostat(10). All measurements were controlled and recorded by a computer.

EXAMPLE 1

The ion conductor of this example consists of 20% Li₂ O, 20% CeO₂, and60% SiO₂ (molar) or molar proportions of 1:1:3. The chemical ingredientsto prepare the ion conductor were 2.758 grams of LiNO₃, 10.965 grams ofammonium-cerium-nitrate, 13.1 ml of TEOS, and 13.1 ml of ethanol. Thewater amount was controlled to produce a molar ratio of water to TEOSequaling approximately 30 (32.4 ml of water). The chemicals weremeasured and mixed and coated as stated above. The resulting ionconductor film was about 500 nm in thickness, and its electro-opticalproperties are shown in FIG. 3. The transmittance (measured at λ=788 nm)depicted in FIG. 3 is for the half-cell including the ion conductor filmdeposited on a 900 nm WO₃ /ITO substrate. The half-cell was repetitivelyoperated (colored and bleached) at ±1.5 volts relative to the restingpotential of WO₃ film, which is 3.2 V above the lithium referenceelectrode. The films were tested using the configuration of FIG. 2. FIG.4 shows the transmittance of the film of Example 1 deposited on a pyrexglass substrate. The solar-weighted transmittance of the film andsubstrate is 87.6%, and the transmittance in the visible range isapproximately 91%. Considering that the visible transmittance of thepyrex substrate by itself is approximately 92%, the high transmittanceof this material makes it ideal for electrochromic applications.

EXAMPLE 2

A 500 nm thick film of the mixed lithium-cerium-silicon oxide wasdeposited by spin coating onto an ITO-coated glass substrate and onto a900 nm WO₃ /ITO substrate. The composition of the mixed oxide was 10%Li₂ O, 30% CeO₂, and 60% SiO₂ (molar).

The electro-optical properties of this mixed oxide ion-conductor filmdeposited on ITO substrate (without any WO₃) were measured by cyclicvoltammetry and shown in FIG. 5. The ion conductor film was sweptbetween ±2 V at a rate of 10 mV/s. Transmittance (λ=788 nm), shown at T,exhibited virtually no change in this voltage range indicating thatinsertion of the Li⁺ ion does not change the optical properties of themixed oxide film itself.

The electro-optical properties of a half-cell containing the same 500 nmmixed-oxide conductor deposited on a 900 nm WO₃ /ITO substrate were alsomeasured by cyclic voltammetry. The ion conductor/WO₃ film was scannedbetween ±1.5 V at 10 mV/s. The resulting recording is shown in FIG. 6.The half-cell was colored showing a decrease in transmittance (λ=788 nm)from approximately 78% to approximately 15%. This shows the ions easilydiffuse through the ion conductor and color the tungsten oxide film.

EXAMPLE 3

A 500 nm thick film of the mixed lithium-cerium-silicon oxide wasdeposited by spin coating onto a 900 nm WO₃ /ITO substrate. Thecomposition of the mixed oxide was 30% Li₂ O, 20% CeO₂, and 50% SiO₂(molar). The electro-optical properties of this half-cell are shown inFIG. 7. This half-cell was repetitively colored and bleached at ±1.5 V.The transmittance was measured at λ=788 nm.

EXAMPLE 4

A 500 nm thick film of the mixed lithium-cerium-silicon oxide wasdeposited by spin coating onto a 900 nm WO₃ /ITO substrate. Thecomposition of the mixed oxide was 30% Li₂ O, 10% CeO₂ 60% SiO₂ (molar).The electro-optical properties of this half-cell are shown in FIG. 8.This half-cell was repetitively colored and bleached at ±1.5 V. Thetransmittance was measured at λ=788 nm.

EXAMPLE 5

A 400 nm thick film of the mixed lithium-cerium-silicon oxide wasdeposited by spin coating onto a 700 nm WO₃ /ITO substrate. Thecomposition of the mixed oxide was 25% Li₂ O, 15% CeO₂, and 60% SiO₂(molar).

An electrochromic device using the lithium-ion conductor has been madewith tungsten oxide as the electrochromic material and vanadium oxide asa lithium-ion storage material. The device has a structure of glass/SnO₂/WO₃ /ion conductor/V₂ O₅ /Au as shown in FIG. 9 although othermaterials can be used for the electrochromic layer, ion storage layer,and the transparent conductor layers as disclosed in a U.S. Pat. No.5,384,653 to Benson et al incorporated by reference. To prepare thecell, SnO₂, 11, is coated on a glass substrate 12. The WO₃ film 13(approximately 600 nm thick) is deposited on the SnO₂ coated substrate.The film 13 was then pre-injected with lithium by thermal evaporation ofa 50 nm thick film of lithium metal onto the WO₃ in a vacuum at 120° C.where lithium spontaneously diffuses into the WO₃ film to form abluish-colored compound Li_(X) WO₃. The ion conductor 14 (approximately500 nm thick) was spin-coated on top the lithiated WO₃ film andthermally treated as set forth above. AV₂ O₅ counter-electrode (15)(approximately 90 nm thick) and a top Au contact (12) (approximately 12nm thick) were then evaporated on the top of the ion conductor in asubsequent operation.

When a positive potential is applied to the colored tungsten oxide(Li_(X) WO₃), lithium ions are moved from the tungsten oxide through thesolid electrolyte into the vanadium oxide and the tungsten oxide changesfrom a partially opaque blue color to transparent. Then, when a negativepotential is applied to the transparent tungsten oxide (WO₃), thelithium ions are moved back from the vanadium oxide through the solidelectrolyte into the tungsten oxide layer and the device returns to thepartially opaque condition.

FIG. 10 shows the changing optical transmittance (measured at wavelengthλ=670 nm) of the typical device of FIG. 9 with an active area of 2.55cm² when operated at ±3 V. The transmittance of the device rapidlychanged from 28% to 9% in less than 10 sec. If the Au top-contact werereplaced by a transparent conductor layer with the visible transmittanceof 85%, the transmittance of the device in the bleached state could beincreased to approximately 55%.

EXAMPLE 6

A reflective electrochromic device was prepared by utilizing the ionconductor film (20% Li₂ O, 20% CeO₂, and 60% SiO₂) of Example 1. Thedevice has a similar structure as shown in FIG. 11. The Au reflectivelayer, 16', is approximately 40 nm thick; the V₂ O₅ layer, 15',approximately 100 nm thick; the ion conductor layer, 14', approximately500 nm thick; and the WO₃ layer, 13', approximately 700 nm thick. TheWO₃ layer 13' was deposited on the SnO₂ 11', glass substrate 12'. Thedevice was prepared in a similar manner as described in the Example 5.

The optical spectra of the reflective electrochromic deviceincorporating the ion conductor of Example 1 and operated at ±3 voltsare shown in FIGS. 12. The solid line 31 in FIG. 12 shows the opticalreflectance of the device in bleached state. The dotted line 32 showsthe optical reflectance of the device in colored state. The light wasincident from the glass side, 12', of the device in FIG. 11. All of theoptical spectra were measured on a Perkin-Elmer 340 spectrophotometer.

Specific examples of electrolyte or conductor material based on the Li₂O--CeO₂ --SiO₂ system have been described above. It is understood thatthe specific examples are representative and that other ratios may beused with good electro-optical characteristics. Specifically, theelectrolyte material can comprise molar percentages of 10%-30% Li₂ O,10%-30% CeO₂, and 50%-60% SiO₂ by molar.

The foregoing descriptions are considered as illustrative only as to theprinciples of the invention. Further, since numerous modifications andchanges will readily occur to those skilled in the art, it is notdesired to limit the invention to the exact construction and process asdescribed above. Accordingly, all suitable modifications and equivalentsmay be resorted to falling within the scope of the invention as definedby the claims as follows.

We claim:
 1. A lithium-ion conducting electrolyte material consisting ofLi₂ O--CeO₂ --SiO₂, wherein CeO₂ is at least approximately 10% by molar.2. The electrolyte material of claim 1 consisting of approximately 20%Li₂ O, 20% CeO₂ and 60% SiO₂ by molar.
 3. The electrolyte material ofclaim 1 consisting of approximately 10% Li₂ O, 30% CeO₂, and 60% SiO₂ bymolar.
 4. The electrolyte material of claim 1 consisting ofapproximately 30% Li₂ O, 20% CeO₂, and 50% SiO₂ by molar.
 5. Theelectrolyte material of claim 1 consisting of approximately 30% Li₂ O,10% CeO₂, 60% SiO₂ by molar.
 6. The electrolyte material of claim 1consisting of approximately 25% Li₂ O, 15% CeO₂, 60% SiO₂ by molar. 7.The electrolyte material of claim 1 consisting of molar percentages of10%-30% Li₂ O, 10%-30%, CeO₂, and 50%-60% SiO₂.
 8. The electrolytematerial of claim 1 in the form of a film.
 9. An ion conductor of amixed oxide consisting of Li₂ O--CeO₂ --SiO₂, wherein CeO₂ is at leastapproximately 10% by molar.
 10. An electrochemical cell comprisinga) twolayers of electrically conductive material; b) a lithium-ion storagelayer between the two layers of electrically conductive material; c) anion conducting layer comprising a Li₂ O--CeO₂ --SiO₂ electrolytematerial between one of the layers of electrically conducting materialand the lithium-ion storage layer.
 11. The cell of claim 10 wherein theion conducting layer comprises approximately 20% Li₂ O, 20% CeO₂ and 60%SiO₂ by molar.
 12. The cell of claim 10 wherein the ion conducting layercomprises 10% Li₂ O, 30% CeO₂ and 60% SiO₂ by molar.
 13. The cell ofclaim 10 wherein the ion conducting layer comprises approximately 30%Li₂ O, 20% CeO₂, and 50% SiO₂ by molar.
 14. The cell of claim 10 whereinthe ion conducting layer comprises approximately 30% Li₂ O, 10% CeO₂,and 60% SiO₂ by molar.
 15. The cell of claim 10 wherein the ionconducting layer comprises approximately 25% Li₂ O, 15% CeO₂, and 60%SiO₂ by molar.
 16. The cell of claim 10 wherein the ion conducting layercomprises molar percentages of 10%-30% Li₂ O, 10%-30% CeO₂, and 50%-60%SiO₂.
 17. The cell of claim 10 wherein the ion conducting layer is afilm.
 18. The cell of claim 10 wherein the two layers of electricallyconductive material are transparent.
 19. The cell of claim 10 whereinone of the layers of electrically conductive material is reflective. 20.The cell of claim 10 further comprising an electrochromic layer betweenone of the layers of electrically conductive material and the ionconducting layer.
 21. A thin film ion conductor positioned between anion storage material and an electrochromic material, said thin film ionconductor comprising oxides of lithium, cerium, and silicon.